New growers question about organics

okay, so im starting an indoor grown inside a PC box.
Blah blah, "PC box is too small etc" ive heard it all... so please dont give me that im just using what ive got available.
because it's a small grow, im going for 12/12 striaght from seed, or perhaps a weeks veg. Im using soil (as i cant afford hydroponics, and mostly they are too large), but i need to know which is the best soil i can get in the UK for this???
also, what is the best nutrients i can get for a soil grow, i dont want to be shovelling shit into it (like molasses and worm castings and that), i just need somthing in a bottle to whack on it and make it grow. is this possible? something cheap and simple???
if anyone has solutions please let me know.
peace.

Ya'll got alot of peat in the moors around there. Thats a start.

well you just need to go to a local grow shop and ask for a good soil to grow tomatos or herbs in. not a weed grow shop but eny will do and sence you have such a small grow in mind then you wont need much. just like a small 4 lt bag.
you will still need to satisfy the plants basic needs N/P/K wise.
you will need 4 things to feed them right.
1. a firt that is high in nitrogen.
2. a firt that is high in potasium.
3. a firt that is high in phosphorus.
4. a micro nute vitamin suplement.
keep in mind that when you combine all of them they need to be as close as can be to even amounts of npk just to keep it as easy as posable for you. it may take a wile to calculate all the firt numbers. hear is an example
1. a firt with n/p/k of 5/3/2 + one with n/p/k of 2/5/3 and one with 3/2/5
will = a firt with a n/p/k of 10/10/10.

To make this short and easy beacuse i could right for hours on this subject i will say that in your situation you can do away with the first step of veg firts due to the short veg you suggested. and sence the plant has enough nutes in the soil for at least 4 weeks you wont need to add alot to begin with just a vitamin supliment. then after you start to see some buds start to form add some not a lot. of the firt that is high in potasium and some that is high in phorphorus. the potasium helps with flower production and the phosphrous helps build strong stems and dence flowers.
and as you sead you want it to be like a put it in a bottel and spray it on tipe of thing so you could just get watter soluble stuf and mix it at half what is tells you to un the lable and spray the soil once every 3 waterings befor you water so it will wash throu to the roots.